为什么第一次点击鼠标的时候不画图？
我为JPanel添加了一个MouseListener监听，当鼠标点击的时候，就在上面画一个圆，现在第一次点击的时候，画完圆就马上消失了，然后再点击就正常了，这是怎么回事？
代码如下：
import java.awt.Graphics;import java.awt.event.MouseEvent;import java.awt.event.MouseListener;import javax.swing.JFrame;import javax.swing.JPanel;public class MouseRe { public static void main(String[] args){ JFrame frame = new JFrame(); QQ a =new QQ(); frame.add(a); frame.setBounds(200, 100, 700, 500); frame.setVisible(true); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); } }class QQ extends JPanel implements MouseListener{ /** * */ private static final long serialVersionUID = 1L; private int i = 30; private int j = 30; public QQ(){ addMouseListener(this); } @Override public void mouseClicked(MouseEvent arg0) { // TODO Auto-generated method stub System.out.println("tadfa"); picture(i,j); i = i + 20; j = j + 20; } @Override public void mouseEntered(MouseEvent arg0) { // TODO Auto-generated method stub } @Override public void mouseExited(MouseEvent arg0) { // TODO Auto-generated method stub } @Override public void mousePressed(MouseEvent arg0) { // TODO Auto-generated method stub } @Override public void mouseReleased(MouseEvent arg0) { // TODO Auto-generated method stub } public void picture(int i,int j){ Graphics g = this.getGraphics(); g.fillOval(i, j, 20,20); }}import java.awt.Graphics;import java.awt.Point;import java.awt.event.MouseEvent;import java.awt.event.MouseListener;import java.util.ArrayList;import java.util.List;import javax.swing.JFrame;import javax.swing.JPanel;public class MouseRe { public static void main(String[] args){ JFrame frame = new JFrame(); QQ a =new QQ(); frame.add(a); frame.setBounds(200, 100, 700, 500); frame.setVisible(true); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); } }class QQ extends JPanel implements MouseListener{ /** * */ private static final long serialVersionUID = 1L; private int i = 30; private int j = 30; private List<Point> dots = new ArrayList<Point>(); public QQ(){ addMouseListener(this); } @Override public void mouseClicked(MouseEvent arg0) { picture(i,j); i = i + 20; j = j + 20; } @Override public void paint(Graphics g) { super.paint(g); for(Point p:dots) g.fillOval(p.x, p.y, 20, 20); } @Override public void mouseEntered(MouseEvent arg0) { // TODO Auto-generated method stub } @Override public void mouseExited(MouseEvent arg0) { // TODO Auto-generated method stub } @Override public void mousePressed(MouseEvent arg0) { // TODO Auto-generated method stub } @Override public void mouseReleased(MouseEvent arg0) { // TODO Auto-generated method stub } public void picture(int i,int j){ dots.add(new Point(i,j)); repaint(); }}